A pin controller is a piece of hardware, usually a set of registers, that
can control PINs. It may be able to multiplex, bias, set load capacitance,
set drive strength, etc. for individual pins or groups of pins.

Definition of PIN:

PINS are equal to pads, fingers, balls or whatever packaging input or
output line you want to control and these are denoted by unsigned integers
in the range 0..maxpin. This numberspace is local to each PIN CONTROLLER, so
there may be several such number spaces in a system. This pin space may
be sparse - i.e. there may be gaps in the space with numbers where no
pin exists.

When a PIN CONTROLLER is instantiated, it will register a descriptor to the
pin control framework, and this descriptor contains an array of pin descriptors
describing the pins handled by this specific pin controller.

Here is an example of a PGA (Pin Grid Array) chip seen from underneath:

To enable the pinctrl subsystem and the subgroups for PINMUX and PINCONF and
selected drivers, you need to select them from your machine’s Kconfig entry,
since these are so tightly integrated with the machines they are used on.
See for example arch/arm/mach-u300/Kconfig for an example.

Pins usually have fancier names than this. You can find these in the datasheet
for your chip. Notice that the core pinctrl.h file provides a fancy macro
called PINCTRL_PIN() to create the struct entries. As you can see I enumerated
the pins from 0 in the upper left corner to 63 in the lower right corner.
This enumeration was arbitrarily chosen, in practice you need to think
through your numbering system so that it matches the layout of registers
and such things in your driver, or the code may become complicated. You must
also consider matching of offsets to the GPIO ranges that may be handled by
the pin controller.

For a padring with 467 pads, as opposed to actual pins, I used an enumeration
like this, walking around the edge of the chip, which seems to be industry
standard too (all these pads had names, too):

Many controllers need to deal with groups of pins, so the pin controller
subsystem has a mechanism for enumerating groups of pins and retrieving the
actual enumerated pins that are part of a certain group.

For example, say that we have a group of pins dealing with an SPI interface
on { 0, 8, 16, 24 }, and a group of pins dealing with an I2C interface on pins
on { 24, 25 }.

These two groups are presented to the pin control subsystem by implementing
some generic pinctrl_ops like this:

The pin control subsystem will call the .get_groups_count() function to
determine the total number of legal selectors, then it will call the other functions
to retrieve the name and pins of the group. Maintaining the data structure of
the groups is up to the driver, this is just a simple example - in practice you
may need more entries in your group structure, for example specific register
ranges associated with each group and so on.

Pins can sometimes be software-configured in various ways, mostly related
to their electronic properties when used as inputs or outputs. For example you
may be able to make an output pin high impedance, or “tristate” meaning it is
effectively disconnected. You may be able to connect an input pin to VDD or GND
using a certain resistor value - pull up and pull down - so that the pin has a
stable value when nothing is driving the rail it is connected to, or when it’s
unconnected.

Pin configuration can be programmed by adding configuration entries into the
mapping table; see section “Board/machine configuration” below.

The format and meaning of the configuration parameter, PLATFORM_X_PULL_UP
above, is entirely defined by the pin controller driver.

The GPIO drivers may want to perform operations of various types on the same
physical pins that are also registered as pin controller pins.

First and foremost, the two subsystems can be used as completely orthogonal,
see the section named “pin control requests from drivers” and
“drivers needing both pin control and GPIOs” below for details. But in some
situations a cross-subsystem mapping between pins and GPIOs is needed.

Since the pin controller subsystem has its pinspace local to the pin controller
we need a mapping so that the pin control subsystem can figure out which pin
controller handles control of a certain GPIO pin. Since a single pin controller
may be muxing several GPIO ranges (typically SoCs that have one set of pins,
but internally several GPIO silicon blocks, each modelled as a struct
gpio_chip) any number of GPIO ranges can be added to a pin controller instance
like this:

So this complex system has one pin controller handling two different
GPIO chips. “chip a” has 16 pins and “chip b” has 8 pins. The “chip a” and
“chip b” have different .pin_base, which means a start pin number of the
GPIO range.

The GPIO range of “chip a” starts from the GPIO base of 32 and actual
pin range also starts from 32. However “chip b” has different starting
offset for the GPIO range and pin range. The GPIO range of “chip b” starts
from GPIO number 48, while the pin range of “chip b” starts from 64.

We can convert a gpio number to actual pin number using this “pin_base”.
They are mapped in the global GPIO pin space at:

chip a:

GPIO range : [32 .. 47]

pin range : [32 .. 47]

chip b:

GPIO range : [48 .. 55]

pin range : [64 .. 71]

The above examples assume the mapping between the GPIOs and pins is
linear. If the mapping is sparse or haphazard, an array of arbitrary pin
numbers can be encoded in the range like this:

In this case the pin_base property will be ignored. If the name of a pin
group is known, the pins and npins elements of the above structure can be
initialised using the function pinctrl_get_group_pins(), e.g. for pin
group “foo”:

When GPIO-specific functions in the pin control subsystem are called, these
ranges will be used to look up the appropriate pin controller by inspecting
and matching the pin to the pin ranges across all controllers. When a
pin controller handling the matching range is found, GPIO-specific functions
will be called on that specific pin controller.

For all functionalities dealing with pin biasing, pin muxing etc, the pin
controller subsystem will look up the corresponding pin number from the passed
in gpio number, and use the range’s internals to retrieve a pin number. After
that, the subsystem passes it on to the pin control driver, so the driver
will get a pin number into its handled number range. Further it is also passed
the range ID value, so that the pin controller knows which range it should
deal with.

Calling pinctrl_add_gpio_range from pinctrl driver is DEPRECATED. Please see
section 2.1 of Documentation/devicetree/bindings/gpio/gpio.txt on how to bind
pinctrl and gpio drivers.

PINMUX, also known as padmux, ballmux, alternate functions or mission modes
is a way for chip vendors producing some kind of electrical packages to use
a certain physical pin (ball, pad, finger, etc) for multiple mutually exclusive
functions, depending on the application. By “application” in this context
we usually mean a way of soldering or wiring the package into an electronic
system, even though the framework makes it possible to also change the function
at runtime.

Here is an example of a PGA (Pin Grid Array) chip seen from underneath:

This is not tetris. The game to think of is chess. Not all PGA/BGA packages
are chessboard-like, big ones have “holes” in some arrangement according to
different design patterns, but we’re using this as a simple example. Of the
pins you see some will be taken by things like a few VCC and GND to feed power
to the chip, and quite a few will be taken by large ports like an external
memory interface. The remaining pins will often be subject to pin multiplexing.

The example 8x8 PGA package above will have pin numbers 0 through 63 assigned
to its physical pins. It will name the pins { A1, A2, A3 ... H6, H7, H8 } using
pinctrl_register_pins() and a suitable data set as shown earlier.

In this 8x8 BGA package the pins { A8, A7, A6, A5 } can be used as an SPI port
(these are four pins: CLK, RXD, TXD, FRM). In that case, pin B5 can be used as
some general-purpose GPIO pin. However, in another setting, pins { A5, B5 } can
be used as an I2C port (these are just two pins: SCL, SDA). Needless to say,
we cannot use the SPI port and I2C port at the same time. However in the inside
of the package the silicon performing the SPI logic can alternatively be routed
out on pins { G4, G3, G2, G1 }.

On the bottom row at { A1, B1, C1, D1, E1, F1, G1, H1 } we have something
special - it’s an external MMC bus that can be 2, 4 or 8 bits wide, and it will
consume 2, 4 or 8 pins respectively, so either { A1, B1 } are taken or
{ A1, B1, C1, D1 } or all of them. If we use all 8 bits, we cannot use the SPI
port on pins { G4, G3, G2, G1 } of course.

This way the silicon blocks present inside the chip can be multiplexed “muxed”
out on different pin ranges. Often contemporary SoC (systems on chip) will
contain several I2C, SPI, SDIO/MMC, etc silicon blocks that can be routed to
different pins by pinmux settings.

Since general-purpose I/O pins (GPIO) are typically always in shortage, it is
common to be able to use almost any pin as a GPIO pin if it is not currently
in use by some other I/O port.

The purpose of the pinmux functionality in the pin controller subsystem is to
abstract and provide pinmux settings to the devices you choose to instantiate
in your machine configuration. It is inspired by the clk, GPIO and regulator
subsystems, so devices will request their mux setting, but it’s also possible
to request a single pin for e.g. GPIO.

Definitions:

FUNCTIONS can be switched in and out by a driver residing with the pin
control subsystem in the drivers/pinctrl/* directory of the kernel. The
pin control driver knows the possible functions. In the example above you can
identify three pinmux functions, one for spi, one for i2c and one for mmc.

FUNCTIONS are assumed to be enumerable from zero in a one-dimensional array.
In this case the array could be something like: { spi0, i2c0, mmc0 }
for the three available functions.

FUNCTIONS have PIN GROUPS as defined on the generic level - so a certain
function is always associated with a certain set of pin groups, could
be just a single one, but could also be many. In the example above the
function i2c is associated with the pins { A5, B5 }, enumerated as
{ 24, 25 } in the controller pin space.

Group names must be unique per pin controller, no two groups on the same
controller may have the same name.

The combination of a FUNCTION and a PIN GROUP determine a certain function
for a certain set of pins. The knowledge of the functions and pin groups
and their machine-specific particulars are kept inside the pinmux driver,
from the outside only the enumerators are known, and the driver core can
request:

The name of a function with a certain selector (>= 0)

A list of groups associated with a certain function

That a certain group in that list to be activated for a certain function

As already described above, pin groups are in turn self-descriptive, so
the core will retrieve the actual pin range in a certain group from the
driver.

FUNCTIONS and GROUPS on a certain PIN CONTROLLER are MAPPED to a certain
device by the board file, device tree or similar machine setup configuration
mechanism, similar to how regulators are connected to devices, usually by
name. Defining a pin controller, function and group thus uniquely identify
the set of pins to be used by a certain device. (If only one possible group
of pins is available for the function, no group name need to be supplied -
the core will simply select the first and only group available.)

In the example case we can define that this particular machine shall
use device spi0 with pinmux function fspi0 group gspi0 and i2c0 on function
fi2c0 group gi2c0, on the primary pin controller, we get mappings
like these:

Every map must be assigned a state name, pin controller, device and
function. The group is not compulsory - if it is omitted the first group
presented by the driver as applicable for the function will be selected,
which is useful for simple cases.

It is possible to map several groups to the same combination of device,
pin controller and function. This is for cases where a certain function on
a certain pin controller may use different sets of pins in different
configurations.

PINS for a certain FUNCTION using a certain PIN GROUP on a certain
PIN CONTROLLER are provided on a first-come first-serve basis, so if some
other device mux setting or GPIO pin request has already taken your physical
pin, you will be denied the use of it. To get (activate) a new setting, the
old one has to be put (deactivated) first.

Sometimes the documentation and hardware registers will be oriented around
pads (or “fingers”) rather than pins - these are the soldering surfaces on the
silicon inside the package, and may or may not match the actual number of
pins/balls underneath the capsule. Pick some enumeration that makes sense to
you. Define enumerators only for the pins you can control if that makes sense.

Assumptions:

We assume that the number of possible function maps to pin groups is limited by
the hardware. I.e. we assume that there is no system where any function can be
mapped to any pin, like in a phone exchange. So the available pin groups for
a certain function will be limited to a few choices (say up to eight or so),
not hundreds or any amount of choices. This is the characteristic we have found
by inspecting available pinmux hardware, and a necessary assumption since we
expect pinmux drivers to present all possible function vs pin group mappings
to the subsystem.

The pinmux core takes care of preventing conflicts on pins and calling
the pin controller driver to execute different settings.

It is the responsibility of the pinmux driver to impose further restrictions
(say for example infer electronic limitations due to load, etc.) to determine
whether or not the requested function can actually be allowed, and in case it
is possible to perform the requested mux setting, poke the hardware so that
this happens.

Pinmux drivers are required to supply a few callback functions, some are
optional. Usually the set_mux() function is implemented, writing values into
some certain registers to activate a certain mux setting for a certain pin.

A simple driver for the above example will work by setting bits 0, 1, 2, 3 or 4
into some register named MUX to select a certain function with a certain
group of pins would work something like this:

In the example activating muxing 0 and 1 at the same time setting bits
0 and 1, uses one pin in common so they would collide.

The beauty of the pinmux subsystem is that since it keeps track of all
pins and who is using them, it will already have denied an impossible
request like that, so the driver does not need to worry about such
things - when it gets a selector passed in, the pinmux subsystem makes
sure no other device or GPIO assignment is already using the selected
pins. Thus bits 0 and 1 in the control register will never be set at the
same time.

All the above functions are mandatory to implement for a pinmux driver.

Note that the following implies that the use case is to use a certain pin
from the Linux kernel using the API in <linux/gpio.h> with gpio_request()
and similar functions. There are cases where you may be using something
that your datasheet calls “GPIO mode”, but actually is just an electrical
configuration for a certain device. See the section below named
“GPIO mode pitfalls” for more details on this scenario.

The public pinmux API contains two functions named pinctrl_gpio_request()
and pinctrl_gpio_free(). These two functions shall ONLY be called from
gpiolib-based drivers as part of their gpio_request() and
gpio_free() semantics. Likewise the pinctrl_gpio_direction_[input|output]
shall only be called from within respective gpio_direction_[input|output]
gpiolib implementation.

NOTE that platforms and individual drivers shall NOT request GPIO pins to be
controlled e.g. muxed in. Instead, implement a proper gpiolib driver and have
that driver request proper muxing and other control for its pins.

The function list could become long, especially if you can convert every
individual pin into a GPIO pin independent of any other pins, and then try
the approach to define every pin as a function.

In this case, the function array would become 64 entries for each GPIO
setting and then the device functions.

For this reason there are two functions a pin control driver can implement
to enable only GPIO on an individual pin: .gpio_request_enable() and
.gpio_disable_free().

This function will pass in the affected GPIO range identified by the pin
controller core, so you know which GPIO pins are being affected by the request
operation.

If your driver needs to have an indication from the framework of whether the
GPIO pin shall be used for input or output you can implement the
.gpio_set_direction() function. As described this shall be called from the
gpiolib driver and the affected GPIO range, pin offset and desired direction
will be passed along to this function.

Alternatively to using these special functions, it is fully allowed to use
named functions for each GPIO pin, the pinctrl_gpio_request() will attempt to
obtain the function “gpioN” where “N” is the global GPIO pin number if no
special GPIO-handler is registered.

Due to the naming conventions used by hardware engineers, where “GPIO”
is taken to mean different things than what the kernel does, the developer
may be confused by a datasheet talking about a pin being possible to set
into “GPIO mode”. It appears that what hardware engineers mean with
“GPIO mode” is not necessarily the use case that is implied in the kernel
interface <linux/gpio.h>: a pin that you grab from kernel code and then
either listen for input or drive high/low to assert/deassert some
external line.

Rather hardware engineers think that “GPIO mode” means that you can
software-control a few electrical properties of the pin that you would
not be able to control if the pin was in some other mode, such as muxed in
for a device.

The GPIO portions of a pin and its relation to a certain pin controller
configuration and muxing logic can be constructed in several ways. Here
are two examples:

Here some electrical properties of the pin can be configured no matter
whether the pin is used for GPIO or not. If you multiplex a GPIO onto a
pin, you can also drive it high/low from “GPIO” registers.
Alternatively, the pin can be controlled by a certain peripheral, while
still applying desired pin config properties. GPIO functionality is thus
orthogonal to any other device using the pin.

In this arrangement the registers for the GPIO portions of the pin controller,
or the registers for the GPIO hardware module are likely to reside in a
separate memory range only intended for GPIO driving, and the register
range dealing with pin config and pin multiplexing get placed into a
different memory range and a separate section of the data sheet.

A flag “strict” in struct pinmux_ops is available to check and deny
simultaneous access to the same pin from GPIO and pin multiplexing
consumers on hardware of this type. The pinctrl driver should set this flag
accordingly.

In this arrangement, the GPIO functionality can always be enabled, such that
e.g. a GPIO input can be used to “spy” on the SPI/I2C/MMC signal while it is
pulsed out. It is likely possible to disrupt the traffic on the pin by doing
wrong things on the GPIO block, as it is never really disconnected. It is
possible that the GPIO, pin config and pin multiplex registers are placed into
the same memory range and the same section of the data sheet, although that
need not be the case.

In some pin controllers, although the physical pins are designed in the same
way as (B), the GPIO function still can’t be enabled at the same time as the
peripheral functions. So again the “strict” flag should be set, denying
simultaneous activation by GPIO and other muxed in devices.

From a kernel point of view, however, these are different aspects of the
hardware and shall be put into different subsystems:

Registers (or fields within registers) that control electrical
properties of the pin such as biasing and drive strength should be
exposed through the pinctrl subsystem, as “pin configuration” settings.

Registers (or fields within registers) that control muxing of signals
from various other HW blocks (e.g. I2C, MMC, or GPIO) onto pins should
be exposed through the pinctrl subsystem, as mux functions.

Registers (or fields within registers) that control GPIO functionality
such as setting a GPIO’s output value, reading a GPIO’s input value, or
setting GPIO pin direction should be exposed through the GPIO subsystem,
and if they also support interrupt capabilities, through the irqchip
abstraction.

Depending on the exact HW register design, some functions exposed by the
GPIO subsystem may call into the pinctrl subsystem in order to
co-ordinate register settings across HW modules. In particular, this may
be needed for HW with separate GPIO and pin controller HW modules, where
e.g. GPIO direction is determined by a register in the pin controller HW
module rather than the GPIO HW module.

Electrical properties of the pin such as biasing and drive strength
may be placed at some pin-specific register in all cases or as part
of the GPIO register in case (B) especially. This doesn’t mean that such
properties necessarily pertain to what the Linux kernel calls “GPIO”.

Example: a pin is usually muxed in to be used as a UART TX line. But during
system sleep, we need to put this pin into “GPIO mode” and ground it.

If you make a 1-to-1 map to the GPIO subsystem for this pin, you may start
to think that you need to come up with something really complex, that the
pin shall be used for UART TX and GPIO at the same time, that you will grab
a pin control handle and set it to a certain state to enable UART TX to be
muxed in, then twist it over to GPIO mode and use gpio_direction_output()
to drive it low during sleep, then mux it over to UART TX again when you
wake up and maybe even gpio_request/gpio_free as part of this cycle. This
all gets very complicated.

The solution is to not think that what the datasheet calls “GPIO mode”
has to be handled by the <linux/gpio.h> interface. Instead view this as
a certain pin config setting. Look in e.g. <linux/pinctrl/pinconf-generic.h>
and you find this in the documentation:

PIN_CONFIG_OUTPUT:

this will configure the pin in output, use argument
1 to indicate high level, argument 0 to indicate low level.

So it is perfectly possible to push a pin into “GPIO mode” and drive the
line low as part of the usual pin control map. So for example your UART
driver may look like this:

Here the pins we want to control are in the “u0_group” and there is some
function called “u0” that can be enabled on this group of pins, and then
everything is UART business as usual. But there is also some function
named “gpio-mode” that can be mapped onto the same pins to move them into
GPIO mode.

This will give the desired effect without any bogus interaction with the
GPIO subsystem. It is just an electrical configuration used by that device
when going to sleep, it might imply that the pin is set into something the
datasheet calls “GPIO mode”, but that is not the point: it is still used
by that UART device to control the pins that pertain to that very UART
driver, putting them into modes needed by the UART. GPIO in the Linux
kernel sense are just some 1-bit line, and is a different use case.

How the registers are poked to attain the push or pull, and output low
configuration and the muxing of the “u0” or “gpio-mode” group onto these
pins is a question for the driver.

Some datasheets will be more helpful and refer to the “GPIO mode” as
“low power mode” rather than anything to do with GPIO. This often means
the same thing electrically speaking, but in this latter case the
software engineers will usually quickly identify that this is some
specific muxing or configuration rather than anything related to the GPIO
API.

Boards and machines define how a certain complete running system is put
together, including how GPIOs and devices are muxed, how regulators are
constrained and how the clock tree looks. Of course pinmux settings are also
part of this.

A pin controller configuration for a machine looks pretty much like a simple
regulator configuration, so for the example array above we want to enable i2c
and spi on the second function mapping:

The dev_name here matches to the unique device name that can be used to look
up the device struct (just like with clockdev or regulators). The function name
must match a function provided by the pinmux driver handling this pin range.

As you can see we may have several pin controllers on the system and thus
we need to specify which one of them contains the functions we wish to map.

You register this pinmux mapping to the pinmux subsystem by simply:

ret = pinctrl_register_mappings(mapping, ARRAY_SIZE(mapping));

Since the above construct is pretty common there is a helper macro to make
it even more compact which assumes you want to use pinctrl-foo and position
0 for mapping, for example:

The mapping table may also contain pin configuration entries. It’s common for
each pin/group to have a number of configuration entries that affect it, so
the table entries for configuration reference an array of config parameters
and values. An example using the convenience macros is shown below:

Finally, some devices expect the mapping table to contain certain specific
named states. When running on hardware that doesn’t need any pin controller
configuration, the mapping table must still contain those named states, in
order to explicitly indicate that the states were provided and intended to
be empty. Table entry macro PIN_MAP_DUMMY_STATE serves the purpose of defining
a named state without causing any pin controller to be programmed:

This example mapping is used to switch between two positions for spi0 at
runtime, as described further below under the heading “Runtime pinmuxing”.

Further it is possible for one named state to affect the muxing of several
groups of pins, say for example in the mmc0 example above, where you can
additively expand the mmc0 bus from 2 to 4 to 8 pins. If we want to use all
three groups for a total of 2+2+4 = 8 pins (for an 8-bit MMC bus as is the
case), we define a mapping like this:

Will be that you activate all the three bottom records in the mapping at
once. Since they share the same name, pin controller device, function and
device, and since we allow multiple groups to match to a single device, they
all get selected, and they all get enabled and disable simultaneously by the
pinmux core.

When a device driver is about to probe the device core will automatically
attempt to issue pinctrl_get_select_default() on these devices.
This way driver writers do not need to add any of the boilerplate code
of the type found below. However when doing fine-grained state selection
and not using the “default” state, you may have to do some device driver
handling of the pinctrl handles and states.

So if you just want to put the pins for a certain device into the default
state and be done with it, there is nothing you need to do besides
providing the proper mapping table. The device core will take care of
the rest.

Generally it is discouraged to let individual drivers get and enable pin
control. So if possible, handle the pin control in platform code or some other
place where you have access to all the affected struct device * pointers. In
some cases where a driver needs to e.g. switch between different mux mappings
at runtime this is not possible.

A typical case is if a driver needs to switch bias of pins from normal
operation and going to sleep, moving from the PINCTRL_STATE_DEFAULT to
PINCTRL_STATE_SLEEP at runtime, re-biasing or even re-muxing pins to save
current in sleep mode.

A driver may request a certain control state to be activated, usually just the
default state like this:

This get/lookup/select/put sequence can just as well be handled by bus drivers
if you don’t want each and every driver to handle it and you know the
arrangement on your bus.

The semantics of the pinctrl APIs are:

pinctrl_get() is called in process context to obtain a handle to all pinctrl
information for a given client device. It will allocate a struct from the
kernel memory to hold the pinmux state. All mapping table parsing or similar
slow operations take place within this API.

devm_pinctrl_get() is a variant of pinctrl_get() that causes pinctrl_put()
to be called automatically on the retrieved pointer when the associated
device is removed. It is recommended to use this function over plain
pinctrl_get().

pinctrl_lookup_state() is called in process context to obtain a handle to a
specific state for a client device. This operation may be slow, too.

pinctrl_select_state() programs pin controller hardware according to the
definition of the state as given by the mapping table. In theory, this is a
fast-path operation, since it only involved blasting some register settings
into hardware. However, note that some pin controllers may have their
registers on a slow/IRQ-based bus, so client devices should not assume they
can call pinctrl_select_state() from non-blocking contexts.

pinctrl_put() frees all information associated with a pinctrl handle.

devm_pinctrl_put() is a variant of pinctrl_put() that may be used to
explicitly destroy a pinctrl object returned by devm_pinctrl_get().
However, use of this function will be rare, due to the automatic cleanup
that will occur even without calling it.

pinctrl_get() must be paired with a plain pinctrl_put().
pinctrl_get() may not be paired with devm_pinctrl_put().
devm_pinctrl_get() can optionally be paired with devm_pinctrl_put().
devm_pinctrl_get() may not be paired with plain pinctrl_put().

Usually the pin control core handled the get/put pair and call out to the
device drivers bookkeeping operations, like checking available functions and
the associated pins, whereas select_state pass on to the pin controller
driver which takes care of activating and/or deactivating the mux setting by
quickly poking some registers.

The pins are allocated for your device when you issue the devm_pinctrl_get()
call, after this you should be able to see this in the debugfs listing of all
pins.

NOTE: the pinctrl system will return -EPROBE_DEFER if it cannot find the
requested pinctrl handles, for example if the pinctrl driver has not yet
registered. Thus make sure that the error path in your driver gracefully
cleans up and is ready to retry the probing later in the startup process.

Here we first request a certain pin state and then request GPIO 14 to be
used. If you’re using the subsystems orthogonally like this, you should
nominally always get your pinctrl handle and select the desired pinctrl
state BEFORE requesting the GPIO. This is a semantic convention to avoid
situations that can be electrically unpleasant, you will certainly want to
mux in and bias pins in a certain way before the GPIO subsystems starts to
deal with them.

The above can be hidden: using the device core, the pinctrl core may be
setting up the config and muxing for the pins right before the device is
probing, nevertheless orthogonal to the GPIO subsystem.

But there are also situations where it makes sense for the GPIO subsystem
to communicate directly with the pinctrl subsystem, using the latter as a
back-end. This is when the GPIO driver may call out to the functions
described in the section “Pin control interaction with the GPIO subsystem”
above. This only involves per-pin multiplexing, and will be completely
hidden behind the gpio_*() function namespace. In this case, the driver
need not interact with the pin control subsystem at all.

If a pin control driver and a GPIO driver is dealing with the same pins
and the use cases involve multiplexing, you MUST implement the pin controller
as a back-end for the GPIO driver like this, unless your hardware design
is such that the GPIO controller can override the pin controller’s
multiplexing state through hardware without the need to interact with the
pin control system.

Pin control map entries can be hogged by the core when the pin controller
is registered. This means that the core will attempt to call pinctrl_get(),
lookup_state() and select_state() on it immediately after the pin control
device has been registered.

This occurs for mapping table entries where the client device name is equal
to the pin controller device name, and the state name is PINCTRL_STATE_DEFAULT:

It is possible to mux a certain function in and out at runtime, say to move
an SPI port from one set of pins to another set of pins. Say for example for
spi0 in the example above, we expose two different groups of pins for the same
function, but with different named in the mapping as described under
“Advanced mapping” above. So that for an SPI device, we have two states named
“pos-A” and “pos-B”.

This snippet first initializes a state object for both groups (in foo_probe()),
then muxes the function in the pins defined by group A, and finally muxes it in
on the pins defined by group B:

The above has to be done from process context. The reservation of the pins
will be done when the state is activated, so in effect one specific pin
can be used by different functions at different times on a running system.